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Full text of "Handbook Of Chemical Engineering - I"

POWER GENERATION AND TRANSMISSION                  37
stokers (American, Jones) the fuel is delivered by a worm or ram to the under side of the fire, and the rising smoke passes through the incandescent fuel. Inclined grate stokers like the Roney (inclined aft) and Murphy (inclined toward the middle, transversely) use a coking plate at the top and an arch overhead, giving a deep furnace, the gas current being more or less downward from the coking plate over the coke fire on the grate.
Stokers require power for operation and if used with coarse or varying sizes of coal will often require the use of a crusher, which consumes an appreciable amount of power. Overhead coal storage, which permits of gravity feed to the stoker hoppers, also greatly increases the boiler-house cost. Coal feeders (for hard coal) are mainly in the experimental stage. They will occasionally save labor cost and possibly slightly improve efficiency by removing the necessity for opening fire doors. Steam jets are sometimes useful in diluting smoke.
Air Supply.—Considering coal as pure carbon, with pure dry air as the agent for supplying oxygen, the maximum temperature is attained when 11.57 Ib. of air are supplied per pound of carbon. If less than this weight of air is supplied, CO will be formed instead of CO2, and the heat evolution per pound of carbon burned will be 4,450 B.t.u. instead of 14,500 B.t.u., for each pound burned to CO instead of C02. If more than 11.57 Ib. of air are supplied, the reactions may be perfect, but the furnace temperature will be reduced. Moreover, since the heat lost to the stack is proportional to the weight of flue gas, this loss of heat increases with excess air supply. A close approximation to the ideal air supply is desirable: the degree of approximation is indicated by the per cent of C02 in the flue gases which is a maximum when the air supply is exactly right.
In practice, some excess of air is found to be necessary, and with amounts too close to the ideal, CO and free 02 may be found present simultaneously. This is to be avoided by thorough distribution of the entering air, avoidance of leaks through furnace and setting walls, and forced draft coupled with rather inadequate suction so as to approximate atmospheric pressure in the furnace (balanced draft). Coals containing hydrogen or hydrocarbons require more air than does pure carbon. Oil requires less air than coal.
Coal Handling and Storage.—The link-belt conveyor is the handling device in most general use, in spite of its high maintenance cost. Traveling cranes with traveling hoists running off on stationary runways, mast and gaff outfits with industrial railways and (for ashes) the pneumatic system may sometimes be considered. In all cases, quick unloading is particularly desirable. This implies a trestle for rail deliveries and a grab-bucket for boats where the water is sufficiently deep to permit the use of large boats. In large plants, ashes should accumulate in a bin over the team driveway, so that carts may be quickly loaded. Coal storage for winter requirements is now generally necessary. There is both shrinkage and deterioration in outside storage, particularly with soft coal. Danger of spontaneous ignition should be met by using ventilating boxes and by having steam and (if possible) some excavating device for extinguishing or digging out the fire when it does occur. Coal weighs 50 to 60 Ib. per cubic foot in its usual commercial forms.
Fuel Oil.—North American fuel oils show a rather steadily progressive increase in density and heat value and decrease in volatile yield as we advance from the East toward the West and Southwest. Eastern oils are paraffin base: